Two common methods of mail integration are via sidebar gadgets and contextual gadgets.

Sidebar gadgets are simple HTML and JavaScript web applications that can be embedded in the left-hand pane of Gmail. These gadgets can extend Gmail with additional functionality and content-rich data displays.

Contextual gadgets draw information from the web based on the contents of an email message. For example, contextual gadgets can display content previews based on links in a message.

Contextual gadgets make use of the following APIs:

Extractor APIs: Contextual gadgets use this API to detect contextual clues in email, determine which types of content trigger the gadget, and pass the triggering content to the gadget. Learn more

IMAP over OAuth API: Gmail supports the IMAP protocol for accessing and sending messages. This API lets the gadget have IMAP access to Gmail via OAuth. Learn more

GMAIL Atom Feeds API: Contextual gadgets use this API to feed updates from websites, blogs, and Gmail. Learn more

If you install an application that uses contextual gadgets, we recommend that you alert your users to the possibility that the gadgets can respond to the content of their email messages.

Domain administrators should evaluate gadgets carefully before installing. Learn more

Apps that integrate with your Google+ have access to your Google+ data, including posts, comments, and circles. The Google+ API allows apps to write posts to the stream and read public posts, domain-restricted posts, posts limited to circles, comments on a post, names of a user’s circles and the membership of each circle, and a merged list of the members of the user’s circles with no circle information. Learn more

Apps that integrate with Single Sign-On allow users to securely sign in to the apps using their G Suite credentials. This can be done in a few ways:

OAuth2 is an open protocol that allows secure authorization in a simple and standard method. All new apps available through the admin console will use Oauth2 for Single Sign-On. Learn more

OpenID Federated Login Service for G Suite allows users to securely sign in to third-party websites using their G Suite user account. Learn more

SAML Single Sign-On allows secure web domains to exchange user-authentication and user-authorization data. Using SAML, an online service provider can contact a separate online identity provider to authenticate users who are trying to access secure content. Learn more

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Unsupported

Marketplace apps that use elements or attributes seen in various HTML pages, Cookies, Javascript, HTTP/S URIs, etc. are not supported by the G Suite APIs Terms of Service.

While we welcome all innovation and integration with our services, note that Marketplace apps which use these implementation types may not always work.

It's important that Google maintain the flexibility to dynamically change our products to provide the service, scale, availability, and innovation our customers expect. If we make substantive changes to a product, we also strive to make the capabilities available that best allow integration with our services.